Semiconductor light-emitting diode

ABSTRACT

A light-emitting diode includes: a semiconductor substrate; and a layered structure, made of an AlGaInP type compound semiconductor material and provided on the semiconductor substrate. The layered structure includes: a light-emitting structure composed of a pair of cladding layers and an active layer for emitting light provided between the pair of cladding layers; and a current diffusion layer which is lattice-mismatched with the light-emitting structure. A lattice mismatch Δ a/a of the current diffusion layer with respect to the light-emitting structure defined by the following expression is −1% or smaller:
 
Δ a/a= ( a   d   −a   e )/ a   e  
 
where a d  is a lattice constant of the current diffusion layer, and a e  is a lattice constant of the light-emitting structure.

This is a continuation of application Ser. No. 09/373,544, filed Aug.13, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light-emitting diode,and more specifically, to a semiconductor light-emitting diode having acurrent diffusion layer.

2. Description of the Related Art

An AlGaInP type material has drawn attention as a material to-be usedfor a light-emitting element which emits light having a wavelength in arange of 550 to 650 nm, since the AlGaInP type material has the largestbandgap of a direct transition type among III–V group compoundsemiconductor materials excluding a nitride. In particular, apn-junction type light-emitting diode, in which a light-emittingstructure (a layered structure including an active layer) made of anAlGaInP type material lattice-matching with GaAs is grown on a GaAssubstrate, is capable of emitting light with higher luminance in awavelength region corresponding to red to green light, as compared witha light-emitting diode provided with a light-emitting structure made ofa material such as GaP or AlGaAs.

In order to form a light-emitting diode with high luminance, it isimportant to enhance a light-emission efficiency as well as a currentinjection efficiency into a light-emitting structure, and to allow lightto efficiently emit from a device.

A conventional light-emitting diode having a light-emitting structuremade of an AlGaInP type material will be described with reference to thedrawings. FIG. 8 is a cross-sectional view of such a light-emittingdiode 200.

As shown in FIG. 8, a light-emitting diode 200 has a structure in whichon an n-type GaAs substrate 61, an n-type GaAs buffer layer 62, alight-emitting structure 69 made of an AlGaInP type material, and ap-type Al_(x)Ga_(1-x)As current diffusion layer 66 are successivelylayered. The light-emitting structure 69 includes an n-type AlGaInPcladding layer 63, a p-type AlGaInP cladding layer 65, and an AlGaInPactive layer 64 interposed between the cladding layers 63 and 65. Ap-type electrode 68 is provided on the top surface of theAl_(x)Ga_(1-x)As current diffusion layer 66, and an n-type electrode 67is provided on the bottom surface of the substrate 61.

A p-type Al_(x)Ga_(1-x)As layer is often used as the current diffusionlayer 66 in such a light-emitting diode 200 as described above for thefollowing reason.

The p-type Al_(x)Ga_(1-x)As layer is transparent to light having awavelength of 550 to 650 nm which can be emitted by the light-emittingstructure 69 made of a (Al_(x)Ga_(1-x))_(y)In_(1-y)P type semiconductormaterial, and therefore advantageous for obtaining a higherlight-emission efficiency. Furthermore, the p-type Al_(x)Ga_(1-x)Aslayer has a low resistivity, which makes it easy to obtain an ohmiccontact with the p-type electrode 68 when employed as the currentdiffusion layer 66. In addition, it is easy to grow a p-typeAl_(x)Ga_(1-x)As layer including crystal of a higher quality, ascompared with an (Al_(x)Ga_(1-x))_(y)In_(1-y)P type semiconductormaterial. Thus, the p-type Al_(x)Ga_(1-x)As layer can be relativelyeasily grown after the growth of a double hetero layer (the “DH layer”)made of an (Al_(x)Ga_(1-x))_(y)In_(1-y)P type, i.e., the light-emittingstructure 69.

Regarding the material to be used for the current diffusion layer 66,comparisons between a conventional Al_(x)Ga_(1-x)As type material and an(Al_(x)Ga_(1-x))_(y)In_(1-y)P type material will be explained below.Throughout the present specification, the term “Al mole fraction” refersto a mole fraction x of Al with respect to Ga (i.e., x=Al/(Al+Ga)). Theterm “In mole fraction” refers to a mole fraction 1−y of In with respectto Al and Ga (i.e., 1−y=In/(Al+Ga+In)). Moreover, the compositions of“(Al_(x)Ga_(1-x))_(y)In_(1-y)P” and “Al_(x)Ga_(1-x)As” may be simplyreferred to as “AlGaInP” and “AlGaAs”, respectively.

FIG. 9 is a graph showing the relationship between the resistivity of anAl_(x)Ga_(1-x)As current diffusion layer lattice-matching with a GaAssubstrate and the Al mole fraction x thereof, and between theresistivity of an (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P current diffusionlayer (i.e., 1−y=0.49) lattice-matching with the GaAs substrate and theAl mole fraction x thereof.

It is understood from FIG. 9 that the Al_(x)Ga_(1-x)As current diffusionlayer exhibits a resistivity of about 0.06 Ω cm, for example, at an Almole fraction x of 0.8. Thus, a low resistivity can be obtained even ata high Al mole fraction x.

In contrast, the (Al_(x)Ga_(1-x))_(0.05)In_(0.49)P current diffusionlayer exhibits a resistivity of about 0.15 to about 3 Ω cm at an Al molefraction x in the range of 0 to 0.8. These values of resistivity arelarger by one order of magnitude than those obtainable with theAl_(x)Ga_(1-x)As layer. Even if the Al mole fraction is decreased, theresistivity is still higher by 50 times than that of theAl_(x)Ga_(1-x)As layer. Accordingly, the(Al_(x)Ga_(1-x))_(0.51)In_(0.49)P current diffusion layer is inferior tothe Al_(x)Ga_(1-x)As current diffusion layer, since a low resistivitycannot be obtained.

Furthermore, in order for the (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P currentdiffusion layer to allow light having a wavelength of 550 to 650 nmemitted from the light-emitting structure 69 to transmit therethrough,it is required to prescribe the Al mole fraction x to be 0.50 or more.In this case, the resistivity of the (Al_(x)Ga_(1-x))_(0.51)In_(0.49)Pcurrent diffusion layer becomes higher by two orders of magnitude, ascompared with that of the Al_(x)Ga_(1-x)As current diffusion layer.

If the resistivity is high, the current diffusion ability of the currentdiffusion layer is decreased, and a current does not spread over theentire chip. As a result, light-emission from a portion of thelight-emitting structure right below the electrode becomes dominant. Thelight emitted from such a portion is likely to be blocked by theelectrode, whereby the emitted light is unlikely to be output.Accordingly, the increase in resistivity of the current diffusion layercauses a light-emission efficiency to decrease. Furthermore, theincrease in resistivity of the current diffusion layer causes anoperating voltage to increase.

Thus, the (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P current diffusion layerwhich is lattice-matched with GaAs has a higher resistivity than that ofthe Al_(x)Ga_(1-x)As current diffusion layer, and consequently hasadverse effects on the operational characteristics of a resultantlight-emitting diode. Therefore, the Al_(x)Ga_(1-x)As layer is typicallyemployed as the current diffusion layer in the conventional art, insteadof the AlGaInP type layer.

As described above, the Al_(x)Ga_(1-x)As layer suffices as the currentdiffusion layer of a light-emitting diode, as far as the resistivity isconcerned. In order for the Al_(x)Ga_(1-x)As current diffusion layer tobe transparent with respect to light having a wavelength of 550 to 650nm, it is required to prescribe an Al mole fraction x thereof to be 0.65or more. However, when the Al mole fraction x becomes high, theAl_(x)Ga_(1-x)As layer will exhibit a deliquescence. Thus, in the casewhere a light-emitting diode having an Al_(x)Ga_(1-x)As layer with ahigh Al mole fraction x is operated under the conditions of hightemperature and high humidity, light intensity is likely to beremarkably decreased.

FIG. 10 shows changes in a chip light intensity (i.e., an intensity oflight obtained from the semiconductor light-emitting diode chip) with apassage of time, in the case where a light-emitting diode chip havingthe Al_(x)Ga_(1-x)As current diffusion layer is operated under theconditions of a temperature of 60° C. and a humidity of 95%. In FIG. 10,data for the chip light intensities are indicated as relative values.

As seen from FIG. 10, as an operating time becomes longer, a chip lightintensity is decreased. Furthermore, as an Al mole fraction becomeslarger, a chip light intensity is more remarkably decreased.

Such a deterioration of a light-emitting diode will be described withreference to FIG. 11. FIG. 11 shows the light-emitting diode 200previously described with reference to FIG. 8, but in a deterioratedcondition. Since like components are designated with like referencenumerals, the explanations thereof are omitted here.

As shown in FIG. 11, while operating the light-emitting diode 200 underthe conditions of high temperature and high humidity, the surface of theAlGaAs current diffusion layer 66 with a high Al mole fraction tends toabsorb moisture so as to be deliquescent, thereby resulting inblack-colored portions 66 a on the surface thereof. Such black-coloredportions 66 a on the surface of the current diffusion layer 66 absorbthe light (represented by arrows in FIG. 11) emitted from the inside ofthe light-emitting diode 200. Thus, in the case where the AlGaAs layerwith a high Al mole fraction is employed as the current diffusion layer,it is difficult to provide a light-emitting diode exhibiting stableluminance over a long period of time.

As described above, although the AlGaAs layer has been typically used asthe current diffusion layer in the conventional semiconductorlight-emitting diode for the reason that a low resistivity can beobtained, the AlGaAs layer is not reliable under the conditions of hightemperature and high humidity. On the other hand, when the(Al_(x)Ga_(1-x))_(0.51)In_(0.49)P layer capable of lattice-matching withthe GaAs substrate typically used is employed as the current diffusionlayer in place of the AlGaAs layer, the resultant current diffusionlayer will then have a higher resistivity, so that sufficient luminancecannot be obtained.

SUMMARY OF THE INVENTION

A light-emitting diode of the present invention includes: asemiconductor substrate; and a layered structure, made of an AlGaInPtype compound semiconductor material and provided on the semiconductorsubstrate. The layered structure includes: a light-emitting structurecomposed of a pair of cladding layers and an active layer for emittinglight provided between the pair of cladding layers; and a currentdiffusion layer which is lattice-mismatched with the light-emittingstructure, wherein a lattice mismatch Δ a/a of the current diffusionlayer with respect to the light-emitting structure defined by thefollowing expression is −1% or smaller:Δa/a=(a _(d) −a _(e))/a _(e)where a_(d) is a lattice constant of the current diffusion layer, anda_(e) is a lattice constant of the light-emitting structure.

Crystal of the semiconductor substrate may be inclined by 8° (8 degrees)to 20° (20 degrees) in a [011] direction with respect to a (100) planethereof.

Preferably, a composition of the current diffusion layer is selected insuch a manner that the current diffusion layer becomes transparent withrespect to a wavelength of light emitted from the light-emittingstructure.

In one embodiment, a composition of the current diffusion layer isexpressed as (Al_(x)Ga_(1-x))_(y)In_(1-y)P, and x is set in the range of0.01 to 0.05 and 1×y is set in the range of 0.01 to 0.30 in thecomposition.

In one embodiment, a composition of the current diffusion layer isexpressed as (Al_(x)Ga_(1-x))_(y)In_(1-y)P and at least one of a valueof x and a value of 1−y in the composition varies along a thicknessdirection of the layered structure.

Both the values of x and 1−y in the composition of the current diffusionlayer may vary, independent of each other.

In one embodiment, a composition of the current diffusion layer isexpressed as (Al_(x)Ga_(1-x))_(y)In_(1-y)P and at least one of a valueof x and a value of 1−y in the composition decreases in a step-likemanner along a thickness direction of the layered structure from aninterface with the light-emitting structure toward an opposite end ofthe current diffusion layer.

Both the values of x and 1−y in the composition of the current diffusionlayer may decrease, independent of each other.

In one embodiment, a composition of the current diffusion layer isexpressed as (Al_(x)Ga_(1-x))_(y)In_(1-y)P and at least one of a valueof x and a value of 1−y in the composition varies in a step-like manneralong a thickness direction of the layered structure from an interfacewith the light-emitting structure toward an opposite end of the currentdiffusion layer, thereby controlling a resistivity of the currentdiffusion layer in the thickness direction.

Both the values of x and 1−y in the composition of the current diffusionlayer may vary, independent of each other.

Thus, the invention described herein makes possible the advantage ofproviding a semiconductor light-emitting diode which has a highluminance and a low resistivity with being transparent to light having awavelength of 550 to 650 nm emitted from a light-emitting structure inthe light-emitting diode, and which does not cause deterioration oflight-emitting characteristics even under the conditions of hightemperature and high humidity.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram illustrating the relationship between the latticeconstant a and the bandgap Eg for various AlGaInP type materials.

FIG. 2 shows graphs illustrating the relationship between theresistivity and the Al mole fraction x of AlGaInP type materials.

FIG. 3 shows graphs illustrating the relationship between theresistivity and the lattice mismatch of AlGaInP type materials.

FIG. 4 is a schematic cross-sectional view of a light-emitting diode inEmbodiment 1 of the present invention.

FIG. 5 shows a graph illustrating changes in a chip light intensity of alight-emitting diode according to the present invention with a passageof time.

FIG. 6 shows a schematic cross-sectional view of a light-emitting diodein Embodiment 2 of the present invention.

FIG. 7A(a) shows a graph illustrating values of the Al mole fraction xat various thickness positions of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer in Embodiment 2.

FIG. 7A(b) shows a graph illustrating values of the In mole fraction 1−yat various thickness positions of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer in Embodiment 2.

FIG. 7B shows a graph illustrating values of the resistivity at thevarious thickness positions of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent diffusion layer in Embodiment 2.

FIG. 8 shows a schematic cross-sectional view of a conventionallight-emitting diode.

FIG. 9 shows a graph illustrating the relationship between theresistivity of AlGaAs type and AlGaInP type current diffusion layers,which are both lattice-matched with a GaAs substrate, and the Al molefraction thereof.

FIG. 10 shows graphs illustrating changes in a chip light intensity ofconventional light-emitting diodes with a passage of time.

FIG. 11 shows a schematic cross-sectional view of the conventionallight-emitting diode of FIG. 8 but in a deteriorated condition.

FIG. 12A shows the [011] direction and the (100) plane relative to asubstrate unit cell.

FIG. 12B is a top view of the crystal substrate showing [011] directionand the (100) plane.

FIG. 12C is a side view of a substrate having an 8° to 20° incline inthe [011] direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The (Al_(x)Ga_(1-x))_(y)In_(1-y)P layer (x=0.50, 1−y=0.49), whichlattice-matches with the substrate and is transparent with respect to alight-emission wavelength, has a higher resistivity than that of theAl_(x)Ga_(1-x)As layer. One reason for this is that a P (phosphorus)group material contained in the AlGaInP layer has a lower mobility thanthat of an As (arsenic) group material contained in the AlGaAs layer,and hence, an effective mass of the AlGaInP layer is large. However,this does not have a significant effect. The more important reason isthat the Al mole fraction x and the In mole fraction 1−y in the(Al_(x)Ga_(1-x))_(y)In_(1-y)P layer lattice-matching with the substrateare high (i.e., x=0.50, 1−y=0.49).

More specifically, if the mole fractions of Al and In of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P layer become high, the AlGaInP layerbecomes more likely to take in oxygen since Al and In are more likely tobe oxidized than Ga. Furthermore, it is difficult to obtain an enhancedpurity for Al and In, as compared with Ga. Therefore, when the Al and Inmole fractions become high, the (Al_(x)Ga_(1-x))_(y)In_(1-y)P layer willcontain a larger amount of impurities such as oxygen and silicon. As aresult, the resistivity of the AlGaInP layer is likely to increase.

Thus, in order to decrease the resistivity of the AlGaInP layer, the Aland In mole fractions therein should be decreased.

However, when the In mole fraction of the AlGaInP layer (to be employedas the current diffusion layer) is decreased, the lattice constantthereof is varied, whereby the AlGaInP current diffusion layer becomeslattice-mismatched with the underlying light-emitting structure.Hereinafter, the relationship between the In mole fraction and thelattice mismatch will be described.

Herein, a lattice mismatch (a lattice mismatch ratio) Δ a/a of thecurrent diffusion layer with respect to the light-emitting structure isdefined by the following expression:Δa/a=(a _(d) −a _(e))/a _(e)where a_(d) denotes a lattice constant of the current diffusion layer,and a_(e) denotes a lattice constant of the light-emitting structure.

As an example, the case where a GaAs substrate is employed is describedbelow.

The lattice constant of GaAs is about 5.65 Å. Since a pair of claddinglayers and an active layer provided therebetween in the light-emittingstructure are successively formed on the GaAs substrate, these layerslattice-match with the GaAs substrate as well as with each other. Thus,the lattice constant of the light-emitting structure is equal to that ofthe GaAs substrate.

FIG. 1 shows the relationship between the bandgap Eg and the latticeconstant a for various AlGaInP type materials.

As shown in FIG. 1, when the In mole fraction is decreased in theAlGaInP type materials (so as to approach to a line connecting AlP andGaP in FIG. 1), the lattice constant of the AlGaInP type materialsgradually becomes smaller than the lattice constant of a GaAs substrate(i.e., that of the light-emitting structure).

The lattice constant of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P currentdiffusion layer is determined by the In mole fraction 1−y thereof. Thelattice mismatch between the (Al_(x)Ga_(1-x))_(y)In_(1-y)P currentdiffusion layer and the GaAs substrate becomes maximum in the case wherethe current diffusion layer contains substantially no In, which exhibitsthe lattice mismatch of about −4%. It is found that such a level of thelattice mismatching will not have a significant effect on a resistivityof a bulk material.

Although the above descriptions are related to the case where the GaAssubstrate is employed, the similar effects can be obtained with anyother appropriate substrates, such as a GaP substrate, an InP substrateand the like. In the case where there is no limit to a material for thesubstrate, the lattice mismatch between the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer and the substratemay become about 8% at most with the variation of the In mole fraction.However, such a lattice mismatch will not have a significant effect on aresistivity of a bulk material.

As described above, although the lattice mismatching is generatedbetween the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer andthe underlying light-emitting structure by decreasing the In molefraction of the constituting material for the current diffusion layer,this will not have significant disadvantages on the characteristics of aresultant light-emitting diode. Thus, by decreasing the In mole fractionof the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer to increasean absolute value of a lattice mismatch in the negative phase, theresistivity of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layercan be reduced. Accordingly, by decreasing the In mole fraction 1−y aswell as the Al mole fraction x of the (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent diffusion layer so that the current diffusion layer becomesintentionally lattice-mismatch with the light-emitting structure, theresistivity of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layercan be prescribed at the same level as that of the conventional AlGaAscurrent diffusion layer. Thus, it becomes possible to form thesatisfactory current diffusion layer even by using the(Al_(x)Ga_(1-x))_(y)In_(1-y)P layer.

FIG. 2 shows data of a resistivity in the case where the Al and In molefractions in various (Al_(x)Ga_(1-x))_(y)In_(1-y)P type materials aredecreased.

As is understood from FIG. 2, the resistivity of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P layer can be remarkably decreased bydecreasing the Al and In mole fractions thereof. In particular, the(Al_(x)Ga_(1-x))_(y)In_(1-y)P layer with the Al mole fraction x of 0.05and the In mole fraction 1−y of 0.05 shows substantially the sameresistivity as that of the AlGaAs current diffusion layer.

The relationship shown in FIG. 2 will be further described in terms ofthe lattice mismatches of the current diffusion layer. In FIG. 3, thehorizontal axis indicates the lattice mismatch Δ a/a of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer with respect tothe substrate and the light-emitting structure, and the vertical axisindicates the resistivity of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P currentdiffusion layer.

It is understood from FIG. 3 that as the absolute value of the latticemismatch becomes larger in the negative phase by decreasing the In molefraction of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer,the resistivity of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusionlayer is also decreased. Since a practical level of resistivity of thecurrent diffusion layer of a light-emitting diode is desirably about 0.1Ω cm or less, it is understood from FIG. 3 that the lattice mismatchbetween the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer andthe underlying light-emitting structure is preferably set to be about−1% or less. In order to realize the lattice mismatch in the aboverange, the In mole fraction 1−y of the (Al_(x)Ga_(1-x))_(y)In_(1-y)Player is required to be about 0.35 or less. Furthermore, it ispreferable that the Al mole fraction x of the (Al_(x)Gal)_(y)In_(1-y)Pcurrent diffusion layer is as low as possible. Accordingly, referringagain to FIG. 3, it is more effective that the Al mole fraction x of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer is prescribed tobe about 0.05 or less.

In the light-emitting diode of the present invention, the latticemismatch of the AlGaInP type current diffusion layer is about −1% orless. Therefore, the AlGaInP type current diffusion layer hassubstantially the same resistivity as that of the conventional AlGaAstype current diffusion layer. Thus, an operating voltage and powerconsumption of the resultant light-emitting diode are not increased, anda high light output efficiency from the light-emitting structure isobtained. Furthermore, the light-emitting diode of the present inventionhas high luminance and is highly reliable.

In a preferred embodiment of the present invention, crystal of thesubstrate is inclined by about 8° (8 degrees) to about 20° (20 degrees)in a [011] direction with respect to a (100) plane (see FIGS. 12A–12C).Therefore, when an (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layerwith a thickness of about 5 to 10 μm which lattice-mismatches with alight-emitting structure is to be grown on a light-emitting structure, ahillock is not generated (which is otherwise generated due to latticemismatching), so that a flat current diffusion layer can be obtained.

In a preferred embodiment of the present invention, the mole fraction ofthe (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer is selected insuch a manner that the current diffusion layer becomes transparent withrespect to a light-emission wavelength of the light-emitting structure.It is important that the current diffusion layer is transparent to alight-emission wavelength of the light-emitting structure and has asufficiently low resistivity. Even in the case where there is latticemismatching, as long as the above-mentioned two characteristics aresatisfied, no disadvantages will arise.

Referring back to FIG. 1, a bandgap of AlGaInP type materials isincreased by decreasing the In mole fraction thereof. Thus, a currentdiffusion layer which is transparent to light having a wavelength ofabout 550 nm to about 650 nm can be formed even without increasing theAl mole fraction thereof.

In a preferred embodiment of the present invention, the Al mole fractionx of the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer ispreferably set to be about 0.01 to about 0.05, while the In molefraction 1−y thereof is preferably set to be about 0.01 to about 0.30.The AlGaInP current diffusion layer with such a small Al fraction isunlikely to be deliquescent. Thus, the light-emission characteristics donot deteriorate even when operated under the conditions of hightemperature and high humidity, unlike the case where the conventionalAl_(0.65)Ga_(0.35)As current diffusion layer is employed.

Thus, according to the present invention, a current diffusion layercapable of being practically used can be produced.

The present invention will be now described by way of illustrativeembodiments with reference to the accompanying drawings. However, thepresent invention is not limited thereto.

Embodiment 1

A semiconductor light-emitting diode exemplified in Embodiment 1 of thepresent invention will be described below with reference to FIG. 4. FIG.4 is a cross-sectional view showing a structure of a light-emittingdiode 100 in Embodiment 1.

As shown in FIG. 4, the light-emitting diode 100 includes an n-type GaAssubstrate 1, a layered structure 12, an n-type electrode 7, and a p-typeelectrode 8. The layered structure 12 includes an n-type GaAs bufferlayer 2, an (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P light-emitting structure11, and a p-type (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer10. The (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P light-emitting structure 11includes an n-type (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P lower claddinglayer 3, an (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P active layer 4, and ap-type (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P upper cladding layer 5. Thep-type electrode 8 is provided on the top surface of the currentdiffusion layer 10, and the n-type electrode 7 is provided on the bottomsurface of the substrate 1.

In the (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P light-emitting structure 11,the mole fractions x in the lower cladding layer 3, the active layer 4,and the upper cladding layer 5 are about 1.0, about 0.3, and about 1.0,respectively. However, the mole fractions x are not limited to thesevalues, and can independently have any value in a range of 0≦x≦1.

In the p-type (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 10,an Al mole fraction x is about 0.05, and an In mole fraction 1−y isabout 0.05.

A method for producing the light-emitting diode 100 in Embodiment 1 willbe described below. The mole fraction of each layer is as describedabove.

The buffer layer 2, the lower cladding layer 3 (thickness: about 1.0μm), the active layer 4 (thickness: about 0.5 μm), and the uppercladding layer 5 (thickness: about 1.0 μm) are successively formed onthe substrate 1 by any known method in the art. The current diffusionlayer 10 (thickness: about 7.0 μm) is then formed on the upper claddinglayer 5 by any known method in the art. Then, an Au—Be film is formed byvapor deposition on the current diffusion layer 10, and patterned to,for example, a circular shape to form the p-type electrode 8. On thebottom surface of the GaAs substrate 1, then-type electrode 7 (e.g.,made of an Au—Zn film) is formed by vapor deposition. Thus, thelight-emitting diode 100 is produced.

Any appropriate method known in the art can be used for forming eachlayer. The electrodes 7 and 8 can have any other appropriate shape andcan be formed by any other appropriate method.

In the light-emitting diode 100 thus produced, the GaAs substrate 1, thebuffer layer 2, the lower cladding layer 3, the active layer 4, and theupper cladding layer 5 are lattice-matched with each other. However, thecurrent diffusion layer 10 lattice-mismatches with these layers. This isbecause the Al and In contents of the current diffusion layer 10 asdescribed above are smaller than the values required for realizing thelattice-matching. The lattice mismatch of the current diffusion layer 10with respect to the underlying light-emitting structure 11 and thesubstrate 1 is about −4% in the above-mentioned structure.Alternatively, the current diffusion layer 10 preferably has a latticemismatch with respect to the underlying light-emitting structure 11 andthe substrate 1 of about −1% or less, more preferably in the range ofabout −4% to about −3%.

Furthermore, the AlGaInP current diffusion layer 10 has a resistivity ofabout 0.1 Ω cm, which is similar to that of a conventional currentdiffusion layer made of AlGaAs.

As described above, the light-emitting diode 100 of the presentinvention is different from the conventional light-emitting diode 200described with reference to FIG. 8 in terms of the material to be usedfor forming the current diffusion layer 10.

Specifically, in the conventional light-emitting diode 200, the currentdiffusion layer 66 is made of an AlGaAs material. Therefore, when thelight-emitting diode 200 is operated under the conditions of hightemperature and high humidity, the surface of the current diffusionlayer 66 becomes deliquescent to form the black-colored portions 66 a(see FIG. 8). As a result, a chip light intensity is deteriorated,whereby the reliability of the conventional light-emitting diode 200 islikely to decrease.

In contrast, in the light-emitting diode 100 of the present invention,the current diffusion layer 10 is made of a p-type(Al_(x)Ga_(1-x))_(y)In_(1-y)P material (e.g., x=0.05, 1−y=0.05). Thus,the Al mole fraction x of the current diffusion layer 10 is small.Accordingly, when the light-emitting diode 100 is operated under theconditions of high temperature and high humidity, the current diffusionlayer 10 does not have deliquescence and does not become black. As aresult, the light-emitting diode 100 of the present invention can bestably operated even with high reliability.

FIG. 5 shows, as reliability data of the light-emitting diode 100 of thepresent invention which has the above-mentioned structure, the chiplight intensity with a passage of time. In FIG. 5, data for the chiplight intensities are indicated as relative values.

It is understood from FIG. 5 that a chip light intensity shows only aslight change (deterioration) during a relatively long period of time upto about 1000 hours under the conditions of a temperature of about 60°C., a humidity of about 95%, and an operating current of about 50 mA.

As described above, in Embodiment 1, the current diffusion layer 10 ismade of an AlGaInP type material. Therefore, a light-emitting diode 100is provided, which has high reliability over a long period of time evenwhen operated under the conditions of high temperature and highhumidity.

In Embodiment 1, by using the current diffusion layer 10 made of AlGaInPcontaining a small amount of Al and In as described above, theresistivity of the AlGaInP current diffusion layer 10 can be prescribedto be as low as that of the conventional current diffusion layer made ofAlGaAs. Thus, a light-emitting diode 100 is provided, which has anAlGaInP current diffusion layer 10 provided with a long currentdiffusion distance from the upper electrode 8 and therefore with a highcurrent diffusion ability. As a result, even when the current diffusionlayer 10 is made of an AlGaInP type material, a light-emitting diode 100having the similar luminance characteristics to those of a conventionallight-emitting diode using an AlGaAs type material can be provided.

Furthermore, as the substrate 1 of the light-emitting diode 100 of thepresent invention, a substrate which is inclined, preferably, by about8° (8 degrees) to about 20° (20 degrees) in a [011] direction withrespect to a (100) plane may be used. Thus, even when the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 10 is alattice-mismatching layer as described above, growth of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 10 is preventedfrom starting from a step which functions as a growth nucleus in acertain orientation. Accordingly, even when a lattice-mismatching(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 10 has a thicknessof about 5 μm to about 10 μm, the flat layer can be grown. Therefore, ap-type electrode 8 of a high quality can be formed on the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 10 with goodcontrollability.

Furthermore, the (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer10 which is transparent to light having a wavelength of about 550 nm to650 nm emitted from the light-emitting structure 11 can be obtained dueto its small amount of In therein.

Embodiment 2

A semiconductor light-emitting diode exemplified in Embodiment 2 of thepresent invention will be described below with reference to FIG. 6. FIG.6 is a cross-sectional view showing a structure of a light-emittingdiode 150 in Embodiment 2.

As shown in FIG. 6, the light-emitting diode 150 in Embodiment 2includes an n-type GaAs substrate 1, a layered structure 12, an n-typeelectrode 7, and a p-type electrode 8. The layered structure 12 includesan n-type GaAs buffer layer 2, an (Al_(x)Ga_(1-x))_(0.51)In_(0.49)Plight-emitting structure 11, an n-type (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent blocking layer 9, and a p-type (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent diffusion layer 106. The light-emitting structure 11 includes ann-type (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P lower cladding layer 3, an(Al_(x)Ga_(1-x))_(0.51)In_(0.49)P active layer 4, and a p-type(Al_(x)Ga_(1-x)) _(0.51)In_(0.49)P upper cladding layer 5. The p-typeelectrode 8 is provided on the top surface of the current diffusionlayer 106, and the n-type electrode 7 is provided on the bottom surfaceof the substrate 1.

In the (Al_(x)Ga_(1-x))_(0.51)In_(0.49)P light-emitting structure 11,the mole fractions x in the lower cladding layer 3, the active layer 4,and the upper cladding layer 5 are about 1.0, about 0.3, and about 1.0,respectively. However, the mole fractions x are not limited to thesevalues, and can independently have any value in a range of 0≦x≦1.

In the n-type (Al_(x)Ga_(1-x))_(y)In_(1-y)P current blocking layer 9, anAl mole fraction x is about 0.30, and an In mole fraction 1−y is about0.49.

Furthermore, in the p-type (Al_(x)Gal)_(y)In_(1-y)P current diffusionlayer 106 of Embodiment 2, the mole fractions are varied along thethickness direction. Accordingly, the p-type(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in thelight-emitting diode 150 of the present embodiment is a graded layer.

FIGS. 7A(a) and 7A(b) show values of the Al mole fraction x and the Inmole fraction 1−y at various positions along the thickness direction(i.e., at the various thickness positions) of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106, respectively.FIG. 7B shows values of the resistivity at the various thicknesspositions of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusionlayer 106. Herein, the thickness direction of the current diffusionlayer 106 is shown in FIG. 6. Thus, in the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in Embodiment2, the In and Al mole fractions are gradually varied from the lowerportion (i.e., from the interface with the light-emitting structure 11)toward the upper portion (i.e., toward the top surface thereof).

A method for producing the light-emitting diode 150 in Embodiment 2 willbe described below. The mole fraction of each layer is as describedabove.

The buffer layer 2, the lower cladding layer 3 (thickness: about 1.0μm), the active layer 4 (thickness: about 0.5 μm), and the uppercladding layer 5 (thickness: about 1.0 μm) are successively formed onthe substrate 1, and the current blocking layer 9 (thickness: about 1.0μm) is further formed on the upper cladding layer 5, in a growth furnaceby any known method in the art. After the substrate 1 having the grownlayers thereon is taken out of the growth furnace, a part of the currentblocking layer 9 is removed by etching so as to be patterned into aprescribed shape. Then, the substrate 1 with the resultant layeredstructure is again set in the growth furnace, and the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 (thickness:about 6 μm) is regrown while gradually varying the Al and In molefractions thereof along its thickness direction (i.e., x=0.20 to 0.01,and 1−y=0.49 to 0.01, as illustrated in FIGS. 7A(a) and 7A(b),respectively). Thereafter, the n-type electrode 7 is formed on thebottom surface of the substrate 1, and the p-type electrode 8 is formedon the graded current diffusion layer 106. The p-type electrode 8 isthen selectively etched away in such a manner that only the portionthereof right above the current blocking layer 9 remains. Thus, thelight-emitting diode 150 is produced.

As described above, the light-emitting diode 150 in Embodiment 2 has thecurrent blocking layer 9. This is advantageous for the following reason.

Light emitted from a portion of the light-emitting structure 11 rightbelow the p-type electrode 8 cannot be taken out since it is blocked bythe electrode 8. Thus, by providing the current blocking layer 9 in thelower portion of the graded current diffusion layer 106 so as to bepositioned right below the p-type electrode 8, a current to be injectedinto the light-emitting structure 11 is allowed to be effectively spreadso as to not flow in the portion right below the p-type electrode 8.Thus, the light-emission from the portion right below the p-typeelectrode 8 is prevented. Accordingly, an invalid current whichotherwise flows into the portion right below the p-type electrode 8 isreduced, and a light-emission efficiency can be improved.

The graded (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 inEmbodiment 2 will be further described with reference to FIGS. 6, 7A(a),7A(b), and 7B.

In an initial growth stage of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent diffusion layer 106, i.e., at a portion of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in thevicinity of the light-emitting structure 11 and the current blockinglayer 9 (in FIG. 6, in the vicinity of a position of about 0 in athickness direction, i.e., in the vicinity of a thickness position ofabout 0), the Al mole fraction x and the In mole fraction 1−y are set tobe about 0.20 and about 0.49, respectively as illustrated in FIGS. 7A(a)and 7A(b). On the other hand, referring again to FIG. 1, a latticeconstant of (Al_(x)Ga_(1-x))_(y)In_(1-y)P is mainly influenced by an Inmole fraction 1−y, not by an Al mole fraction x (or a Ga mole fractioncomplementary to Al mole fraction x). As described above, the In molefraction 1−y of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)P currentdiffusion layer 106 in the vicinity of the thickness position of 0 issubstantially equal to the In mole fraction (about 0.49) of thelight-emitting structure 11 and the current blocking layer 9.Accordingly, the lattice constant of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in an initialgrowth stage thereof is substantially equal to that of thelight-emitting structure 11, and thus, the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 can be grownto be flat. Furthermore, the Al mole fraction x of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in the initialgrowth stage is set at a relatively high value, the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in thevicinity of the thickness position of 0 (i.e., in the vicinity of theinterface with the underlying light-emitting structure 11) is likely tomatch the composition of the underlying upper cladding layer 5; thus,satisfactory crystallinity can be obtained.

The Al and In mole fractions of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)Pcurrent diffusion layer 106 are gradually decreased along the thicknessdirection toward the upper position thereof, whereby both of the Al andIn mole fractions in the uppermost portion (at a thickness position 1 inFIG. 6) of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusionlayer 106 are prescribed to be 0.01.

FIG. 7B shows the corresponding changes in resistivity of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106.

It is understood from FIG. 7B that since the Al and In mole fractionsare high in an initial growth stage, the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 shows aresistivity substantially the same as that of the light-emittingstructure 11. However, the Al and In mole fractions are decreased withthe increase in the thickness, so that a resistivity is also decreased.It should be noted that in the light-emitting diode 150 provided withthe current blocking layer 9, a current is more likely to be spreadthroughout the whole chip through a portion of the current diffusionlayer 106 which has a lower resistivity positioned closer to the p-typeelectrode 8, whereby an operating voltage is less likely to beincreased. On the other hand, even when a resistivity of the currentdiffusion layer 106 in a portion closer to the light-emitting structure11 is relatively high there is no significant influence on, currentdiffusion capability and operating voltage.

As set forth above, in Embodiment 2, the portion of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 in thevicinity of the underlying light-emitting structure 11 has substantiallythe same In mole fraction as those of the light-emitting structure 11and the current blocking layer 9. Accordingly, the lattice constant ofthe portion of the graded current diffusion layer 106 in the vicinity ofthe light-emitting structure 11 becomes substantially equal to those ofthe light-emitting structure 11 and the current blocking layer 9. Thisenhances crystallinity and flatness of a portion of the graded currentdiffusion layer 106 to be grown in an initial growth stage.

Furthermore, the interface of the graded current diffusion layer 106with the light-emitting structure 11 and the current blocking layer 9 isin a satisfactory condition. Moreover, the lattice constant of thegraded current diffusion layer 106 is not rapidly changed since the molefraction thereof changes gradually. Therefore, the improvedcrystallinity and flatness can be obtained throughout the entire currentdiffusion layer 106.

Furthermore, the Al and In mole fractions of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 are decreasedas described above along the thickness direction toward the upperportion thereof. Therefore, the resistivity of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 is allowed togradually decrease in the thickness direction while being kept uniformin a plane parallel to the top surface of the current diffusion layer106. Thus, the injected current can be uniformly spread between theelectrodes 7 and 8.

Accordingly, in Embodiment 2, the improved crystallinity and flatness ofthe (Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 can beobtained without decreasing a light output efficiency or increasing anoperating voltage.

Furthermore, in the case where the Al and In mole fractions are small(i.e., about 1% to about 5%) in an initial growth stage of the(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer, a notch (adiscontinuous portion) in the energy band structure is likely to begenerated at the interface between the current diffusion layer and theupper cladding layer due to a difference in the bandgap energy or theinterface level. Such a notch causes an operating voltage and a drivevoltage to increase. On the other hand, in Embodiment 2, both of thegraded current diffusion layer 106 and the upper cladding layer 5 in thevicinity of the interface therebetween have substantially the same Inmole fractions as each other. Therefore, no notch in the energy bandstructure is generated. Thus, an operating voltage and power consumptionare not increased.

Due to the above-mentioned advantage, the light-emitting diode 150 inEmbodiment 2 can realize light-emission luminance which is about 1.2times that of the conventional light-emitting diode.

In Embodiment 2, the light-emitting diode 150 has been described, inwhich the Al mole fraction x and the In mole fraction 1−y in an initialgrowth stage of the graded (Al_(x)Ga_(1-x))_(y)In_(1-y)P currentdiffusion layer 106 are about 0.20 and about 0.49, respectively.However, the present invention is not limited thereto. Even with thedifferent values of the Al mole fraction x and the In mole fraction 1−y,the same effect as set forth above can be obtained as long as the molefractions of the graded current diffusion layer 106 are varied in thethickness direction.

Moreover, in Embodiment 2, the Al and In mole fractions of the graded(Al_(x)Ga_(1-x))_(y)In_(1-y)P current diffusion layer 106 are graduallyvaried along the thickness direction. However, even when the molefractions may be varied in a step-like manner into two, three, or moresteps, the same effect as set forth above can be obtained.

As described above, the light-emitting diode of the present invention isprovided with the current diffusion layer made of an(Al_(x)Ga_(1-x))_(y)In_(1-y)P material, which is intentionallylattice-mismatched with the underlying light-emitting structure byappropriately selecting the Al mole fraction x and the In mole fraction1−y in the composition thereof. Thus, a light-emitting diode whose lightintensity is not deteriorated even under the conditions of hightemperature and high humidity can be provided without decreasing aresistivity of the current diffusion layer. Furthermore, a light outputefficiency from the light-emitting structure is enhanced, whereby ahighly reliable light-emitting diode can be provided.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

1. A light-emitting diode, comprising: a semiconductor substrate; and alayered structure comprising an AlGaInP type compound semiconductormaterial provided on the semiconductor substrate, the layered structurecomprising: a light-emitting structure comprising of a pair of claddinglayers and an active layer for emitting light provided between the pairof cladding layers; and a current diffusion layer comprising an AlGaInPtype compound semiconductor material, the current diffusion layer beinglattice-mismatched with the light-emitting structure, and whereincrystal of the semiconductor substrate is inclined by 8° (degrees) to20° (20 degrees) in a [011] direction with respect to a (100) planethereof.
 2. The light-emitting diode as in claim 1, wherein a latticemismatch Δa/a of the current diffusion layer with respect to thelight-emitting structure is defined byΔa/a=(a _(d) −a _(e) where a_(d) is a lattice constant of the currentdiffusion layer, and a_(e) is a lattice constant of the light-emittingstructure.
 3. The light-emitting device as in claim 2, wherein thelattice mismatch is −1% or smaller.
 4. A light-emitting diode,comprising: a semiconductor substrate; and a layered structurecomprising an AlGaInP type compound semiconductor material provided onthe semiconductor substrate, the layered structure comprising: alight-emitting structure comprising a pair of cladding layers and anactive layer for emitting light provided between the pair of claddinglayers; a current diffusion layer comprising an AlGaInP type materialwhich is lattice-mismatched with the light-emitting structure and thesemiconductor substrate; and wherein the semiconductor substrate isinclined in a [011] direction with respect to a (100) plane thereof, andwherein crystal of the semiconductor substrate is inclined by 8°(degrees) to 20° (20 degrees) in a [011] direction with respect to a(100) plane thereof.